(1) Field of the Invention
The present invention relates to a bidirectional communication system, and more particularly to a bidirectional communication system in which a number of transmitting time slots and a number of receiving time slots are alternatively set such that data is transmitted to mobile stations by using the transmitting time slots and data from the mobile stations is received by using the receiving time slots.
(2) Description of the Related Art
A TDMA (Time Division Multiple Access) method for creating multiple subchannels for multiple access is known. In TDMA, the duration T, which is called the frame duration, is subdivided into a number, N, of nonoverlapping subintervals (which are called time slots), each of duration T/N. Then each user who wishes to transmit information is assigned to a particular time slot within each frame.
Japanese Laid-Open Patent Application No. 5-102940 discloses a TDMA/TDD technique for use in a TDMA/TDD communication system which utilizes the TDMA (Time Division Multiple Access) method and a TDD (Time Division Duplex) method in combination. The TDD method is disclosed in the above publication.
FIG. 1 shows the way time slots are set within each frame in the TDMA/TDD system of the above publication. In a multiple access system, the channel shared by multiple users is basically partitioned into independent single-user subchannels. In a TDMA/TDD system, the time slots are assigned to available subchannels.
As shown in FIG. 1, in the TDMA/TDD system of the above publication, a number of transmitting time slots Txi (where i=1 to n) and a number of receiving time slots Rxi (where i=1 to n) are alternatively set within each frame. By using the transmitting time slots Txi, a base station transmits information to "n" mobile stations. By using the receiving time slots Rxi, the base station receives information from the "n" mobile stations. In other words, the transmitting time slots Txi involve a corresponding number of burst signals obtained from a multiplexed transmitting signal sent from the base station to the mobile stations, and the receiving time slots Rxi involve a corresponding number of burst signals obtained from a multiplexed receiving signal sent from the mobile stations to the base station.
In the TDMA/TDD system of the above publication, each pair of a transmitting time slot Txi and at receiving time slot Rxi is assigned to one of available subchannels. A maximum amount of data that can be transmitted by using a single time slot is limited. An upper limit of a communication capacity of a TDMA/TDD system is predetermined through communication system design by taking into account the maximum amount of data within a time slot. Therefore, a communication capacity per unit time related to the TDMA/TDD system is limited due to the upper limit of the communication capacity.
However, there is a demand for increasing the communication capacity per unit time related to a single communication system. FIG. 2 shows a problem of the conventional TDMA-TDD system of the above publication. In the case of the TDMA/TDD system of the above publication, in order to increase the communication capacity per unit time, it is necessary that two or more sets of transmitting time slots Txi and receiving time slots Rxi be assigned to one of the available subchannels. If two sets of transmitting time slots Txi and receiving time slots Rxi are assigned to one subchannel, as indicated by shaded lines in FIG. 2, then the number of the available subchannels, or the number of users, that can be simultaneously used is reduced.
Further, when a transmission of a large amount of data in the TDMA/TDD system of the above publication must be started, it is necessary that there exist a certain number of time slots which are not used for another data transmission at that time and enough to transmit the large amount of data. If there are not the unused time slots at that time, some of the used time slots for another data transmission must be forcefully terminated in order to start the transmission of the large amount of data.
It is difficult for the TDMA/TDD system of the above publication to effectively increase the communication capacity per unit time with no need for reducing the number of the available channels.